The present invention relates to an ignition system for internal combustion engines.
In one of conventional ignition systems of this type, such as disclosed in Japanese Laid-Open Patent Publication No. 55-66659, an energy is stored in the primary winding of an ignition coil by the battery voltage and a power transistor is turned on and off by a switching signal thus producing a high voltage across the secondary winding of the ignition coil. A spark is thereby produced at a spark plug through a high-voltage rectifier diode connected to the high-voltage side of the secondary winding.
Then, the current flowing in the primary winding becomes as shown in FIG. 3A and the voltage induced across the secondary winding becomes as shown in FIG. 3B. In this case, the spark plug discharge at a transistor OFF point b shown in FIG. 3A and at this time the ignition coil secondary voltage becomes as shown in FIG. 3C. Also, at this time the discharge current becomes as shown in FIG. 3D. Primarily, the voltage induced in the ignition coil corresponds to a variation of the primary current. Thus, while the voltage for causing the spark plug to discharge is primarily induced at the transistor OFF point b shown in FIG. 3A, a secondary voltage is also induced at a transistor ON point a. This voltage (about 2 KV) corresponds to the ON voltage shown in FIG. 3B. In the conventional ignition system of the distributor-connected type, an idle spark is produced by the distributor high-voltage distributing operation and thus the spark plug is prevented from being fired by the ON voltage. However, in the case of a direct connected-type ignition coil where a spark plug is directly connected to the secondary winding of the ignition coil, no idle spark is produced and there is ample possibility of causing any firing when the transistor is turned on. This causes an engine trouble. As shown in FIG. 3C, this ON voltage is opposite in polarity to the high voltage primarily required for producing the plug spark discharge. Thus, if the high-voltage rectifier diode is connected just before the spark plug, the ON voltage produced at the cathode of the high-voltage rectifier diode no longer appears at the anode of the highvoltage rectifier diode so that there occurs no discharge due to the turning-on of the transistor.
In this type, however, the diode is connected on the high-voltage side between the secondary winding and the spark plug thus giving rise to such problems as breaking down of the high-voltage diode by such capacitive discharge current as shown in FIG. 3D and deterioration of the high-voltage diode due to the discharge producing high voltage.